In liquid crystal displays, a retardation film is generally used in their laminated structure in order to compensate for coloration of their liquid crystal layer or to compensate for viewing angle-dependent changes in retardation. Such an optical material can significantly expand or contract under hot or humid conditions, and such expansion or contraction increases the risk of displacement, lifting or separation. Thus, pressure-sensitive adhesives are required to have durability under such conditions.
In recent years, liquid crystal displays have been reduced in thickness, and pressure-sensitive adhesive layers for use in laminating such a retardation film and the like have been required to be thin enough for a further reduction in thickness. However, such a reduction in thickness has made the durability unsatisfactory.
Up to now, for example, the techniques described below have been proposed to solve the problem of displacement or the like under hot or humid conditions.
There is disclosed a proposal to use an optical member with pressure-sensitive adhesive and an elliptically polarizing plate using the same, in which the affinity of the pressure-sensitive adhesive for both the retardation plate and the polarizing plate is taken into account (see for example Patent Literature 1).
There is disclosed another proposal to use an elliptically polarizing plate having the composition of a pressure-sensitive adhesive layer (1)/a polyvinyl alcohol retardation film/a pressure-sensitive adhesive layer (2)/a polarizing film, wherein the ratio (d1/d2) of the thickness d1 of the pressure-sensitive adhesive layer (1) to that d2 of the pressure-sensitive adhesive layer (2) is from 0.1 to 0.95 (see for example Patent Literatures 2 and 3).
There is disclosed a further proposal to use a pressure-sensitive adhesive comprising 2 to 50% by weight of an imide group-containing monomer copolymerizable with an acrylic monomer (see for example Patent Literature 4).
There is disclosed a further proposal to use an optical pressure-sensitive adhesive using 10 to 20% by weight of an amide group-containing polymerizable monomer (see for example Patent Literature 5).
There is disclosed a further proposal to use a good pressure-sensitive adhesive comprising an acrylamide derivative as a monomer component (see for example Patent Literature 6).
There is disclosed a further proposal to use a pressure-sensitive adhesive comprising, as a main component, 0.5 to 15% by weight of at least one monomer having a carboxyl or amide group and an unsaturated triple bond (see for example Patent Literature 7).
There is disclosed a further proposal to use a pressure-sensitive adhesive obtained by copolymerizing 0.1 to 10 parts by weight of an amide group-containing (meth)acrylic monomer (see for example Patent Literature 8).
There is disclosed a further proposal to use an acrylic pressure-sensitive adhesive comprising 1 to 12% by weight of N-substituted (meth)acrylamide and/or N,N-substituted (meth)acrylamide (see for example Patent Literature 9).
As described above, pressure-sensitive adhesive layers have been required to be thin enough for the display thickness reduction in recent years. However, it has been found that it is difficult for conventional techniques to provide sufficient durability at high temperature or high humidity when the thickness is reduced and that it is difficult for conventional techniques to meet the demand for the thickness reduction.    Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 10-54906    Patent Literature 2: JP-A No. 07-230006    Patent Literature 3: JP-A No. 07-218719    Patent Literature 4: JP-A No. 08-199134    Patent Literature 5: Japanese Patent No. 3645005    Patent Literature 6: JP-A No. 09-176603    Patent Literature 7: JP-A No. 09-113724    Patent Literature 8: JP-A No. 09-288214    Patent Literature 9: JP-A No. 2003-329838